(1) Field of the Invention
The present invention generally relates to an optical deflection apparatus, and more particularly to an optical deflection apparatus in which a deflection angle is continuously controllable.
(2) Description of the Related Art
In recent years, optical switches have been developed, so that miniaturized optical deflection apparatus through which light can be accurately deflected is required for the optical switches.
In general, to deflect the light, a mechanical movable part, such as a galvanomirror, is used. However, because of vibration generated by the mechanical movable part, it is not preferable that a deflection apparatus having the mechanical movable part is used in a precision optical system. Thus, an optical deflection apparatus in which a liquid crystal panel having no mechanical movable part is used is being developed.
An example of a conventional optical deflection apparatus having the liquid crystal is shown in FIG. 1A.
Referring to FIG. 1A, an optical deflection apparatus 41 has a deflection unit 42 and a driving unit 43 for applying a driving voltage to the deflection unit 42. The deflection unit 42 has a pair of glass plates 48 and 49. A transparent electrode 44 which is covered by an orientation film 46 is stacked on the glass plate 48 (see FIG. 1B). Band-shaped transparent electrodes 45 which are covered by an orientation film 47 are stacked on the glass plate 49 (see FIG. 1C). The glass plates 48 and 49 are maintained at predetermined distance by spacers 50, and the orientation films 46 and 47 face each other. A space between the orientation films 45 and 47 is filled with liquid crystal 51. Directions A and A' of the orientation films 46 and 47 are perpendicular to each other (see FIGS. 1B and 1C).
When no voltage is applied across the transparent electrode 44 and the band-shaped transparent electrodes 45, liquid crystal molecules are twisted by the orientation films 46 and 47. In this state, the light does not travel through the liquid crystal 51. When the voltage is applied across the transparent electrode 45 and the band-shaped transparent electrodes 45, the liquid crystal molecules are arranged so that major axes of the molecules are parallel to each other. In this state, the light can travel through the liquid crystal 51.
The driving unit 43 controls voltages applied to electrodes (the band-shaped transparent electrodes 45). When alternate electrodes are provided with voltages as shown in FIG. 2A, stripes arranged at intervals each corresponding to two electrodes are formed. Light incident on the deflection unit 42 is diffracted and deflected by the stripes. The deflection angle is controlled by the intervals of the stripes. When alternate pairs of adjacent electrodes are provided with voltages as shown in FIG. 2B, stripes arranged at intervals each corresponding to two pairs of adjacent electrodes are formed. That is, in this case, the intervals of the stripes are changed, so that the deflection angle is changed.
In addition, it has been proposed that signals having a predetermined frequency and different phases are supplied to the respective electrodes so that a diffraction grating in which the transmittance corresponding to each electrode is gradually varied.
In the above conventional optical deflection apparatus, the voltages applied to the electrodes are controlled so that transmittances of parts, corresponding to the electrodes, of the liquid crystal are controlled. As a result, the pattern of stripes are controlled. Thus, the pattern of tripes arranged at intervals less than the width of each electrode can not be formed, so that it is difficult to obtain a resolution (intervals) corresponding to grade of wavelength of light which resolution is needed to obtain a sufficient deflection angle.
In addition, as shown in FIG. 2B, a space frequency which can be represented by the stripes always corresponds values given by integral multiple of the interval of the stripes. Thus, the space frequency can not be continuously changed, so that the deflection angle can be continuously changed.
Even if the frequencies of the signals supplied to the respective electrodes are changed, the space frequency of stripes corresponding to the electrodes can not be continuously changed.